(1) Field of the Invention
The present invention relates to a secondary cell having a carbon material as a negative electrode and having a non-aqueous electrolytic solution and the producing method thereof.
(2) Description of the Prior Art
As a lithium secondary cell having lithium as a negative electrode retains a high energy density, much research has been carried out in order to realize its practical use. However, lithium utilized in the negative electrode of the lithium secondary cell described above has a problem in that its charge/discharge cycle characteristics deteriorate due to its low charge/discharge efficiency.
In order to solve the above problem, several methods have been proposed, such as utilizing a lithium alloy composed of lithium and aluminum as the material of a negative electrode or utilizing a LiAsF.sub.6 series electrolytic solution in order to enhance the charge/discharge efficiency of lithium and, consequently, improve charge/discharge cycle characteristics. However, in the former method, the voltage of the cell is lowered and in the latter method, the safety of the cell cannot be ensured as the utilized electrolytic solution is noxious.
Japanese Patent Publication Laid-Open No. 62-90863 teaches a method for utilizing a carbon material as the material of a negative electrode.
Although a secondary cell having a carbon system material as the material of a negative electrode solves the problem of lowering the voltage and provides higher safety due to its utilization of no noxious electrolytic solution, its lower energy density becomes a new problem.
A carbon material used as a negative electrode is limited to materials which have a property of desorbing and absorbing lithium. Among the materials with such a property, cokes retain excellent availability. However, cokes have many almost irremovable impurities within themselves. Therefore, in a secondary cell having cokes as the material of a negative electrode, the decomposition of the electrolytic solution occurs, which results in the problem of deteriorating charge/discharge cycle characteristics and storage characteristics.